Solid propellant charge for rocket motors



Jan. 30, 1951 L. M. cRuTcHFll-:LD ETAL soun PRoPELLANT CHARGE FOR ROCKET MoToRs 3 Sheets-Sheet 1 Filed May 28, 1945 N T0 LESLIE M. [CmH/-ILELD W/LL/N E. CAMPBELL Jl?. HARRY H'. MACE Jan. 30, 1951 i.. M. cRuTcHFIELD ETAL 2,539,404

SOLI PROPELLANT CHARGE FOR ROCKET MOTORS Filed May 2s, 1945 s sheets-sheet 2 INVENTo ESL/5 M. muro/1 /ELD w/LL/AM E. GAM/BELL ./R.

By HAR/er t MACE ATTORNEYS Jan. 30, 1951 L. M. cRUTcHFIELD ET AL 2,539,404

soun PROPRLLANT CHARGE RoR ROCKET MoToRs Filed May 28, 1945 3 Sheets-Sheet 3 INVENTOR. LESLIE M. CRUTCHF/ELD W/LUAM E CAMPBELL JR BY l-MHPY W MAGE www;

A7' TORNEYS Pienf'ed im. 3o, 1951 UNITED STATES PATENT OFFICE Leslie M. Crutchleld, Monrovia, William E. Campbell, Jr., Pasadena, and Harry W. Mace, I Glendale, Calif., assignors to Aerojet Engineering Corporation, Azusa, Calif., a corporation ot I Delaware Application May 28, 1945, Serial No. 596,208

(Cl. GII-35.6)

Claims. l

This invention relates to jet propulsion and has for its object to improve the reliability and combustion of solid propellants used in such motors.

A jet or rocket motor comprises a combustion chamber having an exhaust nozzle and a combustible charge in the chamber which upon burning produces gases under high pressure within the chamber which are exhausted at high velocity through the nozzle. Good operation requires that the burning ra'te of the propellant shall be at a steady desired value, as irregularities in burning produce wide changes in pressure which sometimes destroy the motor.

A common practice has been to cast the propellant charge in the chamber by pouring it in at an elevated temperature and allowing it to solidify by cooling. During `the cooling a certain amount of contraction takes place. and furthermore during the wide temperature ranges through which such motors are commonly carried prior to use, expansions and contractions of the propellant occur. This tends to cause cracks in the propellant and pulling away of the propellant from the motor wall resulting in burning in the cracks and down the sides instead of directly across the face of the desired burning layer, thereby appreciably increasing the surface of the propellant burned. Such irregularities in .burning are highly undesirable as they may produce excessiveincreases in pressure.

According to our invention, we 'minimize or prevent such burning irregularities by casting the propellant separate from the motor, for example in the shape of a cylindrical stick, adapted to lit within the motor chamber when the latter is cylindrical. After the casting has solidified we apply to the propellant stick a coating of a thermoplastic material such as hot asphalt, or any other slow burning thermoplastic substance. This coating or liner may be applied by dipping, brushing, spraying or the like and one or more layers may be applied by subsequent applications if desired. After solldiiication of this coating liner we wrap the coated stick with additional material which adheres to the liner and prevents access of flame or hot gases to the sides of the stick on burning. A suitable wrapping material for this purpose is an industrial masking tape or adhesive cloth tape which may for example be applied to the lined stick in an overlapping helical pattern and flreproofed. The lined and wrapped stick may then be inserted into the chamber of the motor.

A preferred feature is an end plate which is 2 placed on the wrapped end of the stick and is secured to the stick by suitable binding.

The foregoing and other features of our invention will be better understood from the following detailed description taken in connection with the accompanying drawings in which:

Fig. 1 is a view of a cylindrical propellant stick lined and wrapped according to our invention;

Fig. 2 shows a partial sectional view of a motor with the linedr and wrapped stick in position;

Fig. 3 shows the manner in which the wrapped stick of Fig. 1 is attached to the end plate;

Fig. 4 shows the shape of the end plate used in Fig. 3; and,

Fig. 5 shows the propellant stick of Fig. 3 in completed form.

In Fig. 1 there is shown a cast propellant in the form of a solid cylinder'or stick I0. This may be composed of any suitable propellant material II containing its own oxidizer, for example an asphalt having mixed in it a suilicient quantity of ground potassium perchlorate or other perchlorate to supply the oxygen for its combustion. The stick may be formed by liquefying the propellant mixture by heating and then pouring it into a suitable mold. The mold in which 4the stick is cast is preferably longer than the 1inished cartridge so as to insure suiiicient material when the finished cartridge is cut to fit the length of the ring chamber of the motor. Upon cooling it solidies in the desired shape.

We then coat the solid .stick with a suitable liner I2 of a thermoplastic material. For this purpose such materials as asphalt, thermoplastic resin or other thermoplastic slow-burning substances may be used. The coating may be done by spraying it on in liquid form or by dipping in the liquid thermoplastic, which then solidifles upon cooling. After the coating has solidified we wrap the coated stick with a wrapping material Ii of a pliable nature which adheres to the liner I2. Cloth or other form of masking tape is suitable for this purpose and may be applied by Wrapping a strip of it helically around the sides of the cylinder, as shown in Figs. 1 and 3, with each turn partially overlapping the previous turn until the entire length of the stick is covered. The end of the stick which will face the closed end of the ring chamber opposite to the exhaust nozzle is likewise covered with strips of tape I3 preferably spaced radially so that they cover the end. Each end cap strip I3 of tape extends a suitable distance down the cylindrical side of the stick. When this end cap is in place the helical winding I6 is wound in reverse start- 3 ing from the uncovered or ring end I5 of the stick and travelling the length of the stick in such s. manner as to cover that portion of the tape forming the end cap which extends down the cylindrical portion of thestick. A suitable rigid plate I1 preferably of metal, circular in shape and having a number of extending ears I8. four in this particular embodiment, is placed on the taped end I9 of the stick.

A series of strips of tape or glass cloth tape 20 are attachedY to one side of the cylindrical wrapped stick. continued across the metal plate and down the opposite side of the stick. These strips of tape or glass cloth 20 extend substantially the entire length of the stick I0. When a sufficient number of these longitudinall strips of tape 20 are attached to the stick so as to insure rigid contact between the end of the stick I9 and the metal plate I1, a third spiral layer of tape 30, shown in Fig. 3, may be helically wound on the stick starting from the steel plate and running the entire length of the stick preferably in the opposite direction from the second layer of tape I6 thereby covering the longitudinal strips 20. Fig. 5 shows the completed charge prepared in the manner described above wherein the wrapping 30 completely covers the anchoring strips 20. A ilreproong solution such as ammonium sulfamate or zinc chloride is then sprayed over the entire taped portion of the stick and permitted to soak into the layers of tape to protect this lining against burning. When the fireproong solution has dried the stick is then covered with a corrugated paper liner 2| which is taped in place. This liner is preferably as long as the tubular section of the firing chamber 23. The stick may then be cut to length by any suitable device such as a wire and the exposed end I5 isgrooved to insure easy starting of the propellant charge upon ignition. After preparing the lined and wrapped stick in this fashion, it may be inserted within the chamber 23 of the jet motor 22 into which the stick Il) has been designed to fit, as shown in Fig. 2. 'I'he motor shown is a cylindrical combustion chamber 23 having an exhaust nozzle 24 at one end, and closed by a cap 25 at the other end, the cap being fitted to the chamber by suitable means such as threads.

The stick is placed in the chamber while the cover 25 is removed with the burning face I5 facing the nozzle 24. To prevent looseness of the stick in the chamber prior to use, a suitable filler member 26 such as a disc of ethyl cellulose, rubber or other thermoplastic material is placed vat the end of the stick facing the closed end of the motor as shown in Fig. 2 so as to press the front end of the stick against the curvature 21 at the nozzle end of the chamber when the cover 25 is screwed on.

The motor is now ready to operateand when desired the burning end I5 of the stick may be ignited by any suitable ignition means such as a conventional ignitor 28 fitted through the nozzle end of the chamber and caused to create an ignition flame 4by the closure of an electric circuit including wires 29. As the particular ignition means used is no part of this invention, no detail of it is given.

In operation when the igniion flame is directed againstthe face I5 of the stick, combustion occurs over the circular surface of the stick producing the volume of gases which are ejected at high velocity out the nozzle 24. Owing to the close adherence and slower burning rate of the liner I2 tothe wall of the stick. no burning on the charge will occur along the side facing the wall of the combustion chamber. This results in steady burning of the circular surface I5 and the burning progresses uniformly down the length of the stick until the stick is consumed.

The metal end plate I1 secures the charge rmly in position when the plate is held against the end of tubular shell 23 by the cushion 26. This device also eliminates any sliding of the charge in a longitudinal direction.

By the use of our inventionthe likelihood of failure due to non-uniform burning is minimized. 'I'he casting of the propellant stick separately from the motor creates the opportunity for a closebinspection of the surfaces for imperfections or irregularities such as bubble holes, cracks and the like which are factors tending to produce burning irregularities. Thus, sticks which are substantially perfect even after their contraction upon cooling can be selected and lined with a liner I2. This liner I2 can then be thoroughly inspected after it is cooled for any irregularities or imperfections, and only those sticks winch are perfectly lined may be selected for wrapping with the outer wrappers I4. Thus the wrapped stick may be placed in the motor with substantial assurance that it is free from defects.A

The asphalt or tar thermoplastic liner I2 maintains its close adherence to the wall of the propellant stick throughout wide variations of temperature such as from a range 20 F. to above F., without breaking away. Assurance is thereby provided that no cracks or spaces along the sides occur in ordinary use which if present might create the burning irregularities resulting in failure.

Our invention has the further advantage that the motor itself may be used manyA times by taking it apart and inserting new propellant sticks which may be supplied for the purpose.

We claim:

1. A propellant charge for a jet motor comprising a solid stick of self-combustible propellant, a thermoplastic liner adhering to the wall of the stick, said liner being of slower burning rate than' that of the propellant, a pliable wrapping around and adhering to said thermoplastic liner and surrounding the stick on all surfaces except on one end thereof, the other end having held against it a rigid plate a portion of which extends beyond lche periphery of the stick, said plate being bound to the end of the stick with pliable tape binding.

2. A propellant charge for a jet motor comprising a solid cylindrical stick of self-combustible propellant, a thermoplastic liner adhering to the cylindrical wall of the stick, said liner being of slower burning rate than that of the propellant, a. pliable wrapping around said thermoplastic liner and surrounding the stick on all surfaces except on one circular end thereof, the other end having held against it a plate having a plurality of projections extending beyond the diameter of the stick, said plate being bound to the end of the stick by a, pliable tape binding.

3. A jet motor comprising a combustion chamber terminating at one end in an exhaust nozzle and at the other end in a cir -ular opening,

a. cap member adapted to enclose the circulary opening of the combustion chamber, a propellant charge in the combustion chamber, said charge comprising a solid stick ofv self-combustible thermoplastic propellant. a thermoplastic coating adhering to the side wall of the stick and a pliable wrapper around the coating, said wrap-y per being maintained in close contact with the wall of the chamber, the exposed end of the stick facing the nozzle, a metal plate with projecting ears secured to the end of the stick remote from the nozzle so that said ears will engage the combustion chamber remote from the nozzle to prevent the propellant charge from sliding in the direction of the exhaust nozzle, and a cushion disposed in the cap member between said metal plate and the closed end of the cap member to prevent the propellant charge from sliding in the direction awayfrom the nozzle.

4. A jet motor comprising a combustion chamber terminating at one end in an exhaust nozzle and at the other endin a circular opening of the combustion chamber,` a cap member enclosing the circular opening', a propellant charge in 'the combustion chamber, said charge comprising a solid stick of self-combustible thermoplastic pro-V pellant having an exposed end, a thermoplastic coating adhering to the side wall of the stick, a pliable wrapper maintained in close contact with the thermoplastic coating, a corrugated liner lining the pliable wrapper, the exposed end of the stick facing the nozzle, a metal plate with projecting ears secured to the end of the stick remote from the nozzle so that said ears will engage the combustion chamber to prevent the propellant charge'from sliding in the direction of the exhaust nozzle, and a cushion disposed in the cap member between the said metal plate and the closed end of the cap member to prevent the propellant charge from sliding in the direction away from the'nozzle.

5. A propellant charge for a jet motor comprising aistick of self-combustible propellant, a thermoplastic liner adhering to the outer peripheral wall of the stick, said liner being of slower burning rate than that of the propellant, a layer of pliable tape wrapped helically around the lined wall, strips of pliable tape carried over one of the ends of the stick and longitudinally along at least part of the wall, a rigid plate over said end and being held in place by at least some of said strips of tape, and another helical layer of pliable tape wound around the wall and covering the first-mentioned helical layer and at least part of said strips,

LESLIE M. CRUTCHFIELD. WILLIAM E. CAMPBELL, JR. .HARRY W. MACE.

REFERENCES CITED -The following references are of record in the ille of this patent:

V UNITED STATES PATENTS Number 

